Nearly all fission reactions utilizing the uranium isotope, U.sub.235, require a concentration of the U.sub.235 isotope greater than in the naturally occurring state. The process of enrichment whereby the concentration of U.sub.235 in natural or depleted uranium is raised to a desired level has been achieved in the past by many techniques which generally operate to separate U.sub.235 from the other uranium isotopes, chiefly U.sub.238, on the basis of its slight chemical or mass difference. Enrichment according to these techniques often requires cascaded processing using a sequence of repeated applications of the same steps, each step providing a slight increase in the concentration of the desired U.sub.235 isotope.
A promising technique for more efficiently separating the U.sub.235 isotope operates by selectively ionizing this isotope in a vapor of uranium without correspondingly ionizing the U.sub.238 isotope in the vapor. The ionization gives to the desired U.sub.235 isotope an electrical characteristic which permits it to be separated from the rest of the components of the uranium vapor. The selective ionization may be provided by illuminating uranium vapor with laser radiations in which at least one laser radiation has a frequency and narrow bandwidth selected to correspond to a specific energy step for the desired U.sub.235 isotope but not the other isotopes of uranium, chiefly U.sub.238. From this intermediate energy level the excited U.sub.235 isotope is illuminated with further laser radiation to cause a transition into the ionized state which has a continuous range of possible energy levels.
In using this technique, it is generally possible that the photon energy in at least one of the laser radiations may or must, depending on the process, exceed one half of the energy of ionization for uranium, approximately 6.2 electron-volts (ev). Two photons from that radiation would then be capable of completely ionizing a uranium atom. In the situation where the radiation having more than half of the ionization energy is not finely tuned in frequency and limited in bandwidth, as is typically the case for the radiation producing the final ionizing step, it is possible to excite significant quantities of the undesired U.sub.238 isotope to a first real, intermediate energy level from which a subsequent photon from the same radiation can produce ionization. In the case where the finely tuned laser radiation has more than one half of the ionizing energy, it is substantially less probable that the U.sub.238 isotope would be excited to a real intermediate energy level, but there is a significant probability for ionizing that U.sub.238 isotope in a two step process employing an intermediate, virtual energy level. Even a low probability for ionization of the undesired U.sub.238 isotope, considering its far greater abundance and the extremely small concentration of the U.sub.235 isotope, can lead to a significant lowering of the separation efficiency.